The present invention relates to methods and compositions for inhibiting the polymerization of vinyl monomers, and more particularly relates, in one embodiment, to methods and compositions for inhibiting the polymerization of vinyl aromatic monomers.
It is well known that undesirable and costly polymerization is a significant problem during the manufacturing of various vinyl monomers, particularly vinyl aromatic compounds, such as styrene. Many kinds of inhibitors have been used in the past to minimize this problem. For instance, inhibitors such as diethylhydroxylamine, phenyl-p-phenylenediamines, tert-butyl catechol, and phenothiazine have been used to control polymer formation. During the early 1980s, compounds selected from the groups called alkyl-substituted di-nitro-phenols and nitroso-phenols found widespread use in the styrene industry. However, because such compounds also functioned as insecticides or were dangerous to handle, their use has been discouraged by environmental and government agencies.
The literature contains references of N,N-disubstituted hydroxylamines as good free radical scavengers, for example diethylhydroxylamine.
Recently, a new class of compounds called stable free radicals is being investigated to replace the nitrophenol products. The literature is replete with examples of xe2x80x9chindered nitroxidesxe2x80x9d as excellent free radical scavengers. Perhaps the most well known such nitroxide used for scavenging free radicals is 2,2,6,6-tetramethyl-1-piperidinyloxy. Although stable free radicals effectively inhibit monomer polymerization, their current cost makes them unattractive. It would be desirable if a composition and method could be devised to overcome some of the problems of the commercial polymerization inhibitors.
Accordingly, it is an object of the present invention to provide a method and composition to effectively inhibit the polymerization of vinyl compounds, such as styrene.
It is another object of the present invention to provide a method and composition to effectively inhibit the polymerization of styrene that is less expensive than using stable free radicals exclusively.
Still another object of the invention is to permit use of a composition to effectively inhibit the polymerization of styrene that has little or no environmental concerns.
In carrying out these and other objects of the invention, there is provided, in one form, a composition for inhibiting polymerization of vinyl compounds comprising a hindered N,N-disubstituted hydroxylamine having the formula:
[(R1R2R3)C]2Nxe2x80x94OH
where R1, R2, and R3 are independently selected from the group consisting of hydrogen, straight, branched or cyclic alkyl, aryl, aralkyl, and alkaryl moieties; where no more than two of R1, R2, and R3 on each C can be hydrogen at a time; where one or more of R1, R2, and R3 on one C may be joined to a R2, R2, and R3 on the other C to form a cyclic moiety selected from the group consisting of alkylene, and aralkylene moieties; where any two of the R1, R2, and R3 on any one C may be joined together to form a cycloalkyl; where any of the above definitions of R1, R2, and R3 may contain one or more heteroatoms selected from the group consisting of N, O and S; and where the total number of carbon atoms in the hindered N,N-disubstituted hydroxylamine ranges from 6 to 70.
It has been discovered that hindered N,N-disubstituted hydroxylamines are an effective treatment to control styrene monomer polymerization. This treatment protocol is most effective when formulated together with synergists such as 2,5-di-tert-butylhydroquinone, 1,2,3,4-tetrahydronaphthalene, and the like.
The hindered N,N-disubstituted hydroxylamines are expected to be useful in inhibiting the polymerization of a variety of vinyl monomers including, but not necessarily limited to, styrene, acrylonitrile, acrylic acid, methacrylic acid, vinyl chloride, acrylates, methacrylates, vinyl ethers, butadiene, isoprene, and the like. It is particularly expected that suitable vinyl aromatic monomers that may be polymerization inhibited by the compositions and methods of this invention include, but are not necessarily limited to, styrene, substituted styrene such as xcex1-methylstyrene, divinylbenzene, vinyltoluene, vinyl naphthalene, and isomers thereof. Preferably, the aromatic monomer is styrene. By xe2x80x9csubstituted styrenesxe2x80x9d is meant styrene substituted with alkyl, aryl, aralkyl, alkaryl hydrocarbon moieties optionally containing oxygen, nitrogen and sulfur, and mixtures thereof. In one non-limiting embodiment, these hydrocarbon substituents have from 1 to 20 carbon atoms, preferably from 1 to 8 carbon atoms, and most preferably from 1 to 4 carbon atoms.
Hindered N,N-Disubstituted Hydroxylamines
Hindered N,N-disubstituted hydroxylamines expected to be effective in the method of this invention include, but are not limited to, those of the formula:
[(R1R2R3)C]2Nxe2x80x94OH
where R1, R2, and R3 are independently selected from the group consisting of hydrogen, straight, branched or cyclic alkyl, aryl, aralkyl, and alkaryl moieties; where only one of R1, R2, and R3 on each C can be hydrogen at a time; where one or more of R1, R2, and R3 on one C may be joined to a R1, R2, and R3 on the other C to form a cyclic moiety selected from the group consisting of alkylene, and aralkylene moieties; where any two of the R1, R2, and R3 on any one C may be joined together to form a cycloalkyl; where any of the above definitions of R1, R2, and R3 may contain one or more heteroatoms selected from the group consisting of N, O and S; and where the total number of carbon atoms in the hindered N,N-disubstituted hydroxylamine ranges from 6 to 70. Preferably, the total number of carbon atoms in the hindered N,N-disubstituted hydroxylamine ranges from 8 to 20, and where the hydrocarbon groups (alkyl aryl, aralkyl, alkaryl, and cyclic moieties) may contain oxygen moieties. In another preferred embodiment of the invention, R1, R2, and R3 independently and individually contain from 1-10 carbon atoms, preferably from 1-8 carbon atoms, more preferably from 1-6 carbon atoms and most preferably from 1-4 carbon atoms. This definition of the hindered N,N-disubstituted hydroxylamines is understood to include structures having functional groups and substituents including, but not necessarily limited to, ether groups, hydroxyl groups, amine groups, carbonyl groups, cyano groups, ester groups, sulfone groups, sulfoxide groups, sulfide groups, and the like.
Some specific, non-limiting examples of suitable hindered N,N-disubstituted hydroxylamines include, but are not necessarily limited to, compounds of the following structures: 
Synthesis of Compounds of Structures X and XI
It is believed that a novel Michael-type addition preparation method for making hindered hydroxylamines of the structure such as X and XI has been discovered. This method has two embodiments, the first of which may be described as Synthesis A: 
where X and Y are hydrogen or the same or different electron withdrawing group and include, but are not necessarily limited to: xe2x80x94NO2, xe2x80x94Cxe2x95x90N, 
where R4, R5, R6, R7, R8, R9, and R11 can independently be the same as R1, R2 and R3, except that both R4 and R5 may be simultaneously hydrogen; and as long as both X and Y are not simultaneously hydrogen, that is, at least one of X or Y must be an electron withdrawing group.
In one example using Synthesis A, R4 and R5 are both methyl and X and Y are cyano.
The second embodiment may be described as Synthesis B: 
where X, Y and R4 and R5 are as above and R10 is xe2x80x94C(R7)3, where R7 is as defined above.
An example of using Synthesis B would be as follows: 
Synergists
Suitable synergists that may optionally be included in the polymerization inhibitor composition of this invention include, but are not limited to substituted alkyl-dihydroxyarenes (e.g., alkylphenols), nitrophenols (including, but not necessarily limited to dinitro-o-cresol and dinitro-sec-butylphenol), hydrogen transfer agents, and the like, and mixtures thereof.
Alkylhydroxyarenes
Suitable substituted alkyl-mono- and alkyl-di-hydroxyarenes for the polymerization inhibiting composition of the invention may include, but are not necessarily limited to, substituted alkylhydroxybenzenes having the formula: 
where R1 and R2 are the same or different and are independently selected from the group consisting of hydrogen, straight, branched, and cyclic alkyl groups averaging from about 1 to about 6 carbon atoms, preferably from about 1 to about 4 carbon atoms, and where x averages from about 1 to about 3, and where y averages from about 1 to 2.
Examples of specific substituted alkylhydroxyarenes that would be suitable in the composition of this invention include, but are not necessarily limited to, 2,6-di-tert-butylphenol; tert-butylhydroquinone; 2,5-di-tert-butylhydroquinone; tert-butylcatechol; hydroquinone; 3,5-di-tert-butylcatechol; catechol; 3,3,3xe2x80x2,3xe2x80x2-tetramethyl-1,1xe2x80x2-spiro-bis-indane-5,5xe2x80x2,6,6xe2x80x2-teterol; and mixtures thereof.
Hydrogen Transfer Agents
It is anticipated that any compound which readily donates hydrogen would be expected to be useful. Hydrogen transfer agents suitable for use in the polymerization inhibiting composition of this invention include, but are not necessarily limited to, naphthalene; anthracene; decalin; hydroquinoline; 1,2,3,4-tetrahydronaphthalene (TETRALIN(copyright); DuPont); 9,10-dihydroanthracene; fluorene; squalane; squalene; tetramethylhydroquinoline; and mixtures thereof.
Proportions
A number of factors affect the effective amount of the hindered hydroxylamines of this invention that would be useful to inhibit the polymerization of a vinyl compound, including, but not necessarily limited to, the nature of the compound, the concentration of the compound, the temperature and pressure environment of the compound, the nature of the particular hindered hydroxylamine used, whether or not a synergist is present, the ratio of the synergists, and the like. The invention is not limited to inhibiting polymerization of vinyl compounds in particular temperature and pressure environments. Nevertheless, some general guidelines as to the effective proportion of the hindered hydroxylamines in the vinyl compound may be given.
For instance, the amount of hindered N,N-disubstituted hydroxylamine in the vinyl compound may range from about 5 to about 1,000 ppm, preferably from about 100 to about 400 ppm, based on the total amount of vinyl compound. If a synergist is employed, the amount of each synergist present ranges from about 0 to about 400 ppm, preferably from about 5 to about 400 ppm, most preferably from about 50 to about 300 ppm, again, based on the total amount of vinyl compound.
If multiple components are employed, the components of the composition may be simply mixed together. They may be mixed together in a single composition prior to addition to the vinyl aromatic compound, although they may also be added to the vinyl compound separately as well.
One non-limiting idea about a possible reaction mechanism of the hindered hydroxylamines of the present invention to inhibit polymerization would be generally shown as follows:
R2Nxe2x80x94OH+R.xe2x86x92R2Nxe2x80x94O.+Rxe2x80x94Hxe2x80x83xe2x80x83(I)
where R. is a growing free radical polymer chain. When the above reaction (I) occurs, the polymerization is inhibited because now polymerization initiation must occur all over again and when it does, the above scavenging reaction can occur again.